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NUMBER 34 

OCCASIONAL PAPERS 

ENGINEER SCHOOL 

UNITED STATES ARMY 

JVV: *1 -. > 



On the Amelioration of Rivers 
With Unstable Beds by 
the System Audouin 


_ 

Translated from the French at the Engineer School , igo8 


WASHINGTON BARRACKS, D. C. 


PRESS OF THE ENGINEER SCHOOL 
1908 

































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3 


/ 3 i 


ON THE AMELIORATION 


OF 


Rivers With Scourable Bottoms 
and Especially of the Loire 


/ Extract from Nouvelles Annales de la Construction 

April, 1904 


FOLLOWED BY A 




PROJECT FOR A TRIAL 


OF THE 


Oblique Dams With Suspended 

Movable Vanes 

(System Audouin) 


BY 

' 

M. CHEMIN 

lngemeur en chef des Fonts et Chaussees en retraite 
Formerly Professor at National School of the Ponts et Chaussees 


i 










n. nt & 

OCT 13 1911 










On the Amelioration of Rivers With Scour- 
able Bottoms and Especially of the Loire. 


OBLIQUE DAM. 

Let us see how these conditions can be satisfied. Let us consider a 
sinuous part of the Loire, and let us suppose that the banks have been 

rectified as explained above. The portions AB, CD are 
regular curves, and there is produced naturally deep hol¬ 
lows from a to b and from c to d. The question is to 
connect these two pools through the sill in order to have 
a continuous channel. Let us place at BE an oblique 
resisting wall: its obliquity will create a locus of exca¬ 
vation. But at the same time it reduces the section of 
flow and the velocity will increase. This action will be 
added to the first; scour will be produced along BE, 
increasing with the angle of the dam and its length. It 
is always possible to chose the angle and length of this 
dam in such manner as to obtain the necessary scour in the time desired. 

DAM WITH SUSPENDED VANES. 

Let us see now how this scour is made and what will become of the 
sand transported. 

It is known that sand transported by the current is carried for the most 
part on the bottom of the river. Particles of earth put in suspension re¬ 
main in greater proportions in the lower layer than in those nearer the sur¬ 
face. (Report of M. Guerard, ingenieur en chef at Marseille, to the 
Congress of Interior Navigation at Paris, 1892.) Hence if, in place of 
making the wall BE by a dam resting on the bottom, it is constructed of 
vanes maintained at a short distance therefrom, there will be created un¬ 
der the dam a strong current, which will produce a rapid scour, carrying 
the sand behind the dam; if this bottom current is reduced sufficiently in 
thickness in relation to the total depth of the water, the velocity will be 
very quickly reduced behind the dam and the sand will be deposited. We 
will be able at will to produce this deposit at a greater or less distance by 




2 


augmenting or diminishing the height of the opening left to the current 
under the vanes. 

As to the phenomenon of the concentration of quantity of movement, 
created by the oblique dam, this will be felt over a zone more or less 
broad along the dam; there will result a scour of the bottom and the sand 
placed in movement, rolling along on this bottom, will fall into the deep 
trough scoured under the dam and will pass in its turn behind this latter. 

The greater part of the sand transported will be deposited behind the 
dam and will form a bar exactly in front of the convex bank, which is its 
natural place, whence the current will not tend to dislodge it even in 
floods, when the vanes are raised; for the dam has been placed precisely 
at the point where the natural force of scour of the current ceases to be 
sufficient of itself. 

A very small quantity of sand will be carried parallel to the dam; it 
will be deposited downstream of the latter in zone encountered outside the 
current, and but a very small quantity will fall into the pool cd , which 
will not be sensibly modified. 

All these phenomena have been verified by an experiment (though 
imperfect) made in the Loire in July, 1897, when the waters reached a 
stage of only 0.80 m. above low water. The scour produced by the cur¬ 
rent passing under vanes slightly raised reached rapidly in a few days a 
depth of about 1 meter. As to the zone in which the concentration of 
quantity of movement was noticeable, it embraced a width of 30 to 40 
meters and the transverse profile possessed a regularity similar to that of 
an artificial canal. 

By operating at average stages—that is to say, at 2 meters above low 
water—the scour would have greater depth; the breadth of the excavated 
zone would be somewhat augmented. 

It is important to remark that this scoured zone is relatively narrow, and 
this is fortunate, because, thus, the sill (crossover) is completely levelled 
down; it is simply crossed by narrow and deep channel, analogous to the 
dredged channels projected by M. Pasqueau for the banks on the Garonne. 
These furrows modify very little the wetted section of the river at low 
water, and, consequently, they have the advantage of giving the greater 
depth of channel without lowering the plane of low water above. 

Thus, when the natural banks of a river present a trace approaching the 
normal trace indicated by M. Fargue, we will be able, by simple inex¬ 
pensive works, to give to the concave banks regular and progressive curves. 
There will then be obtained a continuous channel, always navigable, by 
rectifying the concave banks and placing temporarily near the points of 


inflection an oblique movable dam with suspended vanes capable of regu¬ 
lation. 

This design will have over those now in use the advantage of not pro¬ 
voking the permanent lowering of the crossovers; it will excavate only a 
narrow and deep furrow, modifying but little the wetted section; in con¬ 
sequence, no trouble will be caused upstream while giving at the place of 
improvement a draft of water sufficient for all the needs of navigation. 

It also has the advantage of being more economical than other methods, 
and, in case of floods, forms no obstacle to the free flow of the waters, for 
it is seen, without other explanation, that the vanes can be rapidly taken 
out in such case and there will remain only the piles which support them 
and which will offer but little interference to the floods. 

We will add, also, that the inventor has foreseen the case where it may 
be desirable to leave nothing permanent in the bed of the stream and that 
he has studied a special arrangement to sustain the vanes by floating sup¬ 
ports, which can be transported from point to point in the stream. 

We will not enlarge on the case of a river presenting , several arms or 
branches where one of them only is to be improved. 

It is important to remark that the works of correction of the banks, of 
which we have spoken above, are applied to the natural banks of the river; 
when it becomes necessary to substitute artificial works these should be 
raised to the average height of the banks—that is to say, 3 meters or 3.50 
meters above low water. This is an essential condition to obtain the 
scour desired; for it is only during average stages that the channel can be 
regulated in a permanent manner. M. Jacquet, inspector-general of Fonts 
et Chaussees, has shown in a report to the International Congress of utili¬ 
zation of river waters in 1889: 

11 The action that the waters exert on the bed depends on the discharge, 
and, for each discharge, on its duration. Floods and low waters are not 
those which give to the bars or, nvore generally speaking, to the bed of 
the stream, the forms of relief seen at low water. The effect produced by 
great floods is modified by the less high waters that follow and which de¬ 
termine the definitive form that persists after each flood. These inter¬ 
mediate stages, above low water, are really regulators of the low-water 
bed, and it is on them that we must base our study of the limit of action 
on the created minor bed.” 

Low works too long submerged are, moreover, proscribed in the scheme 
before us. 

As soon as an oblique wall is submerged it ceases to be a strong cause 


4 


of scour. This fact has been observed very clearly on the Loire in 1897, 
notably above Ingrandes. 

RESUME. 

To sum up, the design of M. Audouin for the amelioration of rivers 
with erodible bottom consists essentially in the rectification of the banks 
of the mean bed, completed at need by plunging spurs joined to the con¬ 
vex shore, a permanent amelioration which is only partial and insufficient 
in low waters, and to complete this amelioration by the creation of an arti¬ 
ficial channel through the crossings, but only during low water. 

The means proposed to create this furrow constitutes in itself an inno¬ 
vation of general interest. It may be found of advantage for the improve¬ 
ment of rivers emptying into tideless seas. The sort of decantation and 
storing of the sedimentary matter that it can cause all along the course of 
an important river of this nature-would have for subsidiary effect to retard 
in certain measure the advance of the delta by diminishing the supplies 
which go to form it. 

This is a new and very simple idea, of which the importance will escape 
no one. 


Project for Oblique Dams With Suspended 

Movable Vanes. 

(System Audouin.) 

The project that is presented here has for its aim to demonstrate the 
practical working of dams with suspended vanes (system Audouin) for the 
improvement of rivers with erodible bottoms. 

All engineers known the difficulties presented by this problem. For 
years the trials of all sorts that have been made to solve this problem have 
been almost sterile, and it may be said that it is only since the researches 
and works of Inspector-General Fargue that we are able to determine the 
conditions under which improvements ought to be attempted and that we 
have been able to carry them out effectively. The examples of the Rhone 
and the great German rivers show what success may be expected. 

However, certain rivers present very exceptional difficulties. Among 
these particularly is the Loire. Here is found a major bed limited by 
dikes, that must be made insubmersible, between which ought to be theo¬ 
retically contained the waters of the great floods, and which must not be 
overflowed or broken under penalty of great disasters in the neighboring 
low lands they are designed to protect. 

The establishment in the bed thus constituted of permanent works which 
would concentrate the waters in a determined bed beforehand can not be 
considered; for they would, as a consequence, diminish the section of flow, 
already scarcely sufficient during extremely high floods. On the other 
hand, during the period of low water, the discharge is reduced so much 
that navigation becomes impossible if all the discharge can not be confined in 
a minor bed sufficiently restricted in order that this small quantity of water 
may nevertheless assure the passage of the boats which frequent the river 
and render their passage of the crossings possible. 

It results from these numerous conditions that the solutions adopted 
elsewhere and which have given satisfactory results meet here difficulties 
of a quite special order and that it is practically impossible to predict 
whether or not their use would be crowned with success. 

The system designed by the late Captain Audouin, of which a practical 
trial is now proposed, would appear to prove efficacious where these other 
methods could succeed only with great difficulty or imperfectly. The 
principles on which it rests are, in sum, only happy and reasonable appli- 


6 


cations of those which M. Fargue has formulated. We will not develop 
them here and will refer for their exposition to the brochure in which they 
are clearly described.* But before speaking of their application, it will be 
well to develop some general considerations, attaching more especially to 
the Loire. 

When one looks over the successive plans of soundings of this river exe¬ 
cuted at different epochs and with the greatest care by the engineers of the 
Special Service of the Loire, one can not fail to recognize, among the inces¬ 
sant secondary changes that the waters impress on the form of the bottom, 
certain characteristics which appear to remain constant or which vary within 
small limits. It may be stated, in passing, that the laws of M. Fargue find 
here their verification. The pools or hollows are found along the concave 
banks, the bars are at the crossovers between two pools—that is to say, at 
the points of inflection which separate two consecutive curves, concave in 
opposite sense. The bed of the river is then formed of a series of echelons 
of which the pools and the crossings constitute the steps. 

In order to ensure navigability at low water, it is then necessary to render 
possible the passage from one pool to the next; but it must be done with 
the condition that the stage of the upstream pool is not sensibly changed 
by the works executed. Without this, the improvement of the difficult 
passage will have, as a consequence, to aggravate the condition of the region 
upstream and therefore render more difficult the works needed there. The 
difficulty is only moved from one locality to another. 

If the discharge at low water were always sufficiently great, it might be 
possible, by uniting and concentrating all the water, to diminish to a cer¬ 
tain extent these undesirable effects without, however, escaping them alto¬ 
gether. But as this is not the case on the Loire, it follows logically that 
the crossings to improve must, naturally, present the depth necessary for 
navigation and have sufficient breadth to assure at all times the easy pas¬ 
sage of boats, but that it w r ould be imprudent, and even be inimical to the 
end in view, to try to obtain at these points channels of greater breadth 
than is strictly necessary, for the normal low water will not be sufficient to 
fill them and consequently no local improvement will be realized, while 
having, moreover, aggravated the general condition. 

From w r hat precedes, the end to be attained is then to form, from one 
pool to the next, channels of breadth and depth strictly sufficient to these 
desiderata by means of works that do not obstruct the bed of the river 
when in flood. 

*Esquisse d’un nouveau systeme d’amelioration des rivieres a fond mobile, parL. 
Audouin, capitaine d’artillerie brevete. Paris, Librairie Polytechnique Ch. Beranger, 
editeur. 





It is this double result that will follow from a proper application of the 
Audouin System. The experiments made at Montjean in 1897, under 
conditions quite unfavorable, have nevertheless demonstrated that the 
creation of a channel under conditions that have been described above has 
been easily and rapidly realized. A certificate of the Foreman of the 
Society of Miners and Quarrymen of Montjean gives testimony from a 
practical point of view. The prohles drawn up by M. Audouin establish it 
equally. But as these facts have lacked official sanction in consequence 
of an absence of control, the Society of Amelioration of the Navigability 
of Rivers, which has been created to make known and exploit the Audouin 
System, has thought that a new trial, surrounded this time with all desirable 
guarantees, would establish, in an indisputable manner, the advantages 
claimed for this system. It is to carry out these views and in accord with 
the Service des Ponts et Chaussees the present project has been drawn up. 

The locality has been selected, by common agreement, in the Loire, on 
the left bank a little above the mouth of the Maine. The works 
will commence a little below B. M. 558. The bank presents at this 
point a pool with a curvature sufficiently great to retain the channel con¬ 
stantly along it with a good depth of water. The dam w'ill continue the 
curve of the bank across a bar through which is to be excavated the 
channel, and will be directed towards the following pool located at the 
downstream end of the lie aux Dames. 

The case is one which Engineer-in-Chief Girardon calls “a bad cross¬ 
ing for in plan the two pools overlap and the passage from one to the 
other (when it is possible) is made actually in following a direction almost 
normal to that of the bed of the stream. 

The dam, of which the location and general direction have been fixed 
in concert wfith the Engineers of the Service of the Loire, will have a total 
length of 500 meters. The first part, 250 meters in length, constitutes a 
rectification of the left bank which it prolongs into the enlarged bed at 
this point, and which it joins tangentially. It will be made of wattlings 
held by piles having 3.5 meters penetration, driven 1.8 meters apart, with 
small piles between with 1.5 meters penetration. 

The second part wfill also be 250 meters in length. It will be formed 
of piles of .25 m. average diameter, 7 to 7.5 meters in length, 1.8 meters 
apart, and will be driven so that their heads will have a uniform height of 
2 meters above low water. They wfill have a penetration of from 4.5 to 
5 meters; they will be united at their tops by a double course of waling. 
They wfill also be connected by a lower waling piece, held on by hoops, 
2 meters below the upper waling. It will be placed under water. Each 
element will be 5.7 meters in length in order to be supported by four 


8 


consecutive piles. These waling pieces support the guides to receive and 
guide the vanes. 

In order to make provision for the inevitable irregularities in driving 
piles, these guides, spaced .9 m. apart, will be placed between adjoining 
piles and will descend to the bottom or near it. They will receive the 
vanes, which will themselves be made in two parts, one fixed, the 
other movable, both being .8 m. in breadth. The fixed vane, 2.5 meters 
long, placed in the first guide, will be held at a fixed height by means of 
iron hooks fixed in its upper part. It will not be moved and will be 
withdrawn only when a flood is of sufficient magnitude to require removal 
of the dam in order to bring no obstacle to the free escape of the water. 

The movable part, 1.6 meters in height, will move in other guides 
superposed on the first, and arranged so that the posterior face may move 
along the anterior face of the fixed vane. A rod, attached at the middle, 
permits the movable vane to descend as scour below is produced in order 
to preserve under it a sheet of water whose momentum continues to scour 
the bottom and to transport the soil to the rear, as has been explained in 
the memoirs of Captain Audouin. For this first trial the vanes, both 
fixed and movable, have been designed in wood. Later, for extended 
operations, they may be made of sheet iron. 

The placing and maneuvering of both kinds of vanes will ordinarily be 
done by means of boasts moored in rear of the line of piles. The material 
carried by the Loire is composed almost solely of sand which travels along 
the bottom and enters into suspension only in the larger floods; the chok¬ 
ing up of the guides, rendering maneuvers difficult or impossible, is there¬ 
fore not to be feared. It is to be noted that these maneuvers will always 
be very easy from the small size of the vanes; if by chance they can not 
be made by hand, the normal manner, they can always be made by means 
of levers or windlass supported on the upper wales. 

As already stated, the total removal of the vanes is only necessary in 
high floods when their presence in the major bed would form an obstacle 
likely to cause dangerous perturbations. The rest of the time they should 
remain in place until the desired channel is obtained and fixed in position. 

It is recognized, of course, that one of the results of the establishment 
of the system considered will consist in the creation behind the vanes of a 
bar of which the height will grow successively up to a variable figure, in 
each case depending on local circumstances. 

This bar should be protected and fixed progressively by the means ordi¬ 
narily in use—spurs, sunken mattresses, plantations, etc. As it follows 
very closely the form of the dam and the latter determines the trace of the 
channel created, it is seen that it will constitute in some sort the bank of 


9 


the channel thus obtained, and, once properly fixed in position, it will con¬ 
tinue to assure its direction and regimen when the dam is removed to be 
utilized elsewhere. 

When the stream is in flood and the dam is submerged, it will not prove 
a danger to navigation, for the piles will serve to carry buoys which will 
mark their location and indicate to boatmen if navigation is still possible. 

It is to be remarked that in this system, by utilizing the mean stage, the 
channel will be scoured out in a relatively short time; it will then be fixed 
in position. On the contrary, by the employment of submerged spurs, it 
will be only after a considerable time that the channel becomes useful, 
even if the depths desired are realized. In the former the realization is 
certain and, so to speak, immediate. 

We may be permitted to emphasize the fact that the trial now in view 
has not for its immediate object to transform into a good crossing all the 
bar existing downstream of B. M. 558. The resources at our disposal do 
not permit undertaking this work in its whole extent. What we seek, 
especially, is to demonstrate that the system possesses the merits claimed 
and that its use at difficult localities furnishes the solution that other sys¬ 
tems can not supply, at least with the same rapidity and security. 

The estimated expense is 22,000 francs. We should ask the help of the 
state in order to make this trial, which interests to so high a degree all the 
country traversed by the Loire, an exhaustive one. We do not doubt but 
that it would be granted, as was done for the more important trials which are 
now in course of execution between the mouth of the Maine and Cha- 
lonnes. 

Angers , February Q, IQ 05 . 

O. Chemin, 

Ingenieur en chef des Fonts et Chaussees en retraite , 

Ancien profesjeur a VEcole nationale des Ponts et Chaussees. 


* 



Sketch of a New Method of 
Improving Rivers With 
Erodible Bottoms 

BY 

L. AUDOUIN 

Captain of Artillery 






































































Sketch of a New Method of Improving Rivers 

With Erodible Bottoms. 


EXPLANATION OF PRINCIPLES. 

We will take as the basis of our description the Loire between 
Nantes and the Maine. 

The end to be attained is to assure the existence of a continuous 
channel in which is maintained, even during the periods of lowest 
water, a depth of water sufficient for navigation. 

Considering that the discharge of the Loire at low water is only 
120 to 150 cubic meters between Angers and Nantes, while the 
breadth of the river varies from 350 to 550 meters and more, it is 
evident that the channel can have a sufficient depth of water only on 
the condition that a species of minor bed of restricted width is formed. 

To obtain this minor bed, we employ different methods, according 
as the river separates into several branches or, on the contrary, flows 
in a single bed. Where the river separates into several branches, we 
choose one of them in which to direct the channel and we regulate 
the entrance to this branch so as to concentrate therein the volume 
of water necessary to the existence of a good channel, while directing 
the sand towards the other branches. Where the river has only a 
single branch of considerable width we cause the formation of the 
minor bed by the execution of suitable works. 

UTILIZATION OF THE FORCE OF THE CURRENT. 

The current itself will be employed to dig its channel. The force 
thus brought in play was studied many years ago by M. Fargue, whose 
experiments lead to conclusions which may be summarized as follows: 

"Every resisting surface struck more or less obliquely by the current 
becomes a center of concentration of momentum, and, if the bottom 
is erodible, also of scour. It, therefore, follows in a winding river 
that the channel will always be excavated along the concave bank. 



14 


"But the form of this bank is not immaterial; it must present a 
regular curve without abrupt change, and the succession of curves 
must be properly adapted to the width of the bed and the velocity of 
the current. At the points of inflection of the current are always 
found corresponding shoals which may be made to disappear by con¬ 
tracting the bed at these points.” 

These principles are evidently applicable to the Loire, the bottom 
of which is as erodible as possible, since it is formed of a very fine sand. 
If,therefore,the banks were regulated, following regular and progressive 
curves adapted to the case in hand, the channel would be excavated 
along the concave banks, and would always form at these points a 
depth of water sufficient for navigation. But shoals would occur at 
the points of inflection and the depth of water on these shoals resulting 
from the forming of the sand bars would be insufficient for navigation 
during the low water unless a contraction of the bed was effected at 
the points of inflection. 

These contractions of the bed, on the one hand, have the advantage 
of removing, at the point considered, the obstacle which the bar op¬ 
poses to navigation, but on the other, they present very grave objections. 
They result always in a lowering of the low-water plane, and conse¬ 
quently aggravate the difficulties of navigation on the bars higher up 
the river. They have in fact only suppressed an obstacle at one point, 
and caused it to reappear in an aggravated form a little higher up. 
Moreover, on the Loire the contraction of the bed has presented par¬ 
ticular objections because the violence of the floods in this river, 
already so terrible, would be rendered still more dangerous. 

The regulation of the banks is thus incapable of producing a com¬ 
plete improvement of the conditions of navigability of a river. But 
this regulation of the banks has nevertheless the advantage of assuring 
a permanent improvement by giving to the thalweg a direction more 
or less constant passing from concave bank to concave bank, and a 
depth which is insufficient only on the bars, and only during the 
period of low water. To complete this improvement, it would, there¬ 
fore, be sufficient to find a means of creating, during the period of 
low water, an artificial channel through the bars. 

In order that this requisite may be satisfied it is necessary and suffi¬ 
cient that the stream be permitted to dig through the bar, fairly rapidly, 
a channel of suitable depth in order that navigation may be always 
possible; it is also necessary that the creation of this channel shall 


15 


not result in material modification of the channel above and below, 
and does not sensibly change the level of the water. 

OBLIQUE DAM. 


Let us see how these conditions may be satisfied. Consider a 
winding portion of the Loire represented in Fig. 1, and suppose that 

the banks have been regulated as described above; the 
portions ab, cd are laid out in regular curves, and deep 
basins, or pools, are naturally produced extending from 
ab and from cd. It is a question of connecting these 
two pools through the bar in orderto form acontinuous 
channel. Place at BE an oblique resisting wall; its 
obliquity will produce a center of scour. But at the 
same time it contracts the section of flow, and the 
velocity will increase. This action being added to 
the first, scour will take place along BE at a rate 
increasing with the obliquity and length given to the dam. It will 
always be possible to choose the direction and length of this dam 
so as to obtain the necessary scour in the time desired. 



DAM WITH SUSPENDED GATES. 

Let us see now how this scour will be effected and what will become 
of the sand carried along in the water. 

It is known that sand held in suspension by a current drifts generally 
along the bottom of the river. If particles made from lemon are put 
in suspension they rest in layers, decreasing in density from the bottom 
to the surface of the water. 

Therefore if, in place of making the wall BE a dam resting on 
the bottom, it is made with gates held a small distance above the 
bottom, there will be established under the dam a very strong current 
which will produce a rapid scour, carrying the sand behind the dam. 
If this bottom current is reduced to a depth quite small as compared 
to the total depth of water it will spread very quickly in the mass of 
calm water behind the dam, and the sand will be deposited. We 
might at our pleasure cause a deposit of the sand at a greater or less 
distance by increasing or diminishing the height of the passage left 
for the current under the gates. 

As to the phenomenon of the concentration of momentum caused by 
the oblique dam, it will make itself felt in a zone more or less wide 
along the dam, there will result a scour along the bottom, and the 


16 


sand put in motion, rolling along the bottom, will fall in the deep 
excavation made under the dam, and it will pass in its turn behind 
the dam. 

Thus the great proportion of the suspended sand will finally be 
deposited behind the dam, and will form a strand exactly in front of 
the convex bank, which is its natural place, from which the current 
will have no tendency to dislodge it, even in high water, when the 
gates of the dam will be removed, for the dam has been placed exactly 
at the point where the force of scour of the current is no longer suf¬ 
ficient to cause motion of the sand. 

A very small quantity of the sand will be carried parallel to the dam ; 
it will be deposited at the end of the dam in the zone outside the in¬ 
fluence of the current and it will flow gradually into the pool cd , which 
will not be appreciably modified. 

All these phenomena have been verified in an actual experiment 
(rather imperfect, however) carried out on the Loire in July, 1897, 
when the water was only To of a meter above the low-water stage. 
The scour caused by the passage of the current under a slightly ele¬ 
vated gate caused rapidly in a few days an increase of depth of approxi¬ 
mately one meter. As to the zone in which the concentration of 
momentum made itself felt, it reached a breadth of 30 to 40 meters, 
and the profile through it showed a regularity similar to that of an 
artificial canal. 

By operating when the river was at the two-meter stage, the scour 
would be deeper, and the breadth of the scoured zone would be some¬ 
what increased. 

It is important to note that this scoured zone is quite narrow, and 
this is a fortunate circumstance, because the bar is not thus entirely 
removed; it is simply crossed by a narrow deep channel similar to the 
dredged channels recommended by M. Pasqueau, Inspector General 
of Roads and Bridges, for the work on the Garonne. These channels 
do not much modify the wetted perimeter of the river during low 
water, and they have consequently the advantage of giving more depth 
without causing a lowering of the plane of low water. 

Thus, when the natural banks present a direction which resembles 
the normal direction (or trace) indicated by M. Fargue, it would be 
possible by simple and inexpensive works to give regular and progres¬ 
sive curves to the concave banks. Consequently, a continuous channel, 
always navigable, will be obtained by regulating the concave banks 
and by placing temporarily near points of inflection an oblique mov¬ 
able dam with suspended gates possible of regulation. 


17 


This method will have, over those actually in use, the advantage 
of not causing the permanent removal of the bars; it will dig in them 
only a narrow deep channel, modifying but little the wetted perimeter, 
and consequently not disturbing the condition of the river above, 
while giving at the point improved a depth of water greater than by 
any other method. 

It will have at the same time the advantage of being more eco¬ 
nomical than other methods. 

But it will not always happen that the plan of the natural banks 
allows us to give to the concave banks a regular curve sufficient to 
cause a scour giving a channel sufficiently deep. The experiments 
of M. Fargue have shown that in this case the channel indeed has a 
tendency to form along the concave bank; but is not maintained there 
in as regular a manner, and it has not a sufficient depth. These ob¬ 
jections might be remedied by the employment of spurs attached to 
the convex bank. These spurs result in controlling the windings of 
the channel, and maintaining it along the concave bank; at the same 
time they retain the sand, causing a rise in the bottom along the con¬ 
vex bank, and consequently contract the section of flow: whence an 
increase in velocity and in the scouring force, which can be exercised 
only irf the unprotected zone, causes the desired scour along the concave 
bank. 

We see by the addition of these spurs that the principles proposed 
by M. Fargue become of a more general application, and we can 
always have along the concave bank the necessary depth of water. 
We may then make the channel across from one bank to the other, 
assuring the passage over the bar by the aid of a dam with suspended 
and regulated gates. 

Such are the principles on which depend the works which we pro¬ 
pose to use in localities where the river has only one channel. 

******* 

CHOICE OF NAVIGABLE CHANNEL. 

The choice of the channel which must be reserved for navigation 
will be determined by considerations of two kinds: first, it is neces¬ 
sary that the breadth and configuration of this channel be well adapted 
to the maintenance of a good channel; in the second place it is 
necessary that the entrance of this channel be oriented and arranged so 
that the thalweg naturally directs itself to it in high water as well as 
in low water. 

As to the first class of considerations, experience shows that narrow 


18 


channels whose width is in the neighborhood of a hundred meters and 
rather uniform, are those in which the channel is best maintained. 
To obtain, in such branches, a channel which will always be good, a 
few works of regulation on the banks to assure the regularity of the 
curves and uniformity of width will be sufficient. 

To realize the conditions indicated by the second class of considera¬ 
tions, it might be necessary to make at the entrance to the navigable 
arm and above, rather considerable regulation works on the banks in 
order to insure, in all cases, the direction of the thalweg toward the 
entrance or the arm or branch chosen for improvement. 

The advantages of utilizing narrow arms are sufficiently important 
to justify the execution of very considerable regulation works on the 
banks; for this utilizing of narrow arms will permit the establishment 
of transverse dams which may serve at the same time as ground sills for 
the regulation of the longitudinal profile, and as reservoir dams for 
raising the level of the water and for the creation of hydraulic motive 
power. 

REGULATION OF BANKS. 

The dam with suspended gates may be employed for the execution 
of regulating works on the banks under the conditions which will be 
indicated further on. It seems that the employment of this system of 
dams should procure the advantage of promptly reclaiming certain 
portions of land cutoff from the bed by regulation. This rapid recla¬ 
mation might become for the state a source of considerable profit if 
care is taken, as was done before the execution of the works on the 
Maritime Seine, to have decided, by law, that the land thus re¬ 
claimed shall be the property of the state and not of the riparian 
inhabitants. 

* * * * * * * 

GENERAL ORDER OF EXECUTION OF THE WORKS. 

In order that the results of the employment of the proposed system 
may be appreciated, a complete scheme of the order of execution of 
the works must be presented. 

First of all the banks must be regulated, following a reasonable 
plan: on the one hand, the attempt must be made to give to the con¬ 
cave banks the progressive form recognized as most favorable, and on 
the other care must be taken not to diverge so far from the natural 
forms of the bed as to diminish the importance of the works to be 
executed. 


19 


The work of bank regulation consists in replacing an irregular con¬ 
cave bank with an artificial bank of gradual curvature placed most 
often in front of the former bank. 

This artificial bank is formed by a dam with suspended gates 
w r hich will cause a rapid filling in of the portion cut off from the 
river bed. As soon as this filling in has attained a sufficient height 
the dam will be moved and the new bank will be covered with riprap. 

This filling in offers important advantages. It furnishes a means 
of getting rid of sand put in motion by works of regulation, and 
which will thus be definitely thrown outside of the river bed. More¬ 
over, if this filling can be raised to a height approximating that of 
the banks it will give a considerable extent of land which may be again 
put under cultivation and will have an important value. 

It seems that this filling may be obtained rapidly by the employment 
of the dam with suspended gates, since the presence of this oblique 
dam causes the sand, which the scouring force carries forward, to pass 
immediately behind the dam. In whatever way this phenomenon 
may be produced, there is necessarily a difference of level on the two 
faces of the dam; therefore the water passing under the gates must 
find a sufficient opening for its discharge. It will not be necessary to 
raise the gates along the entire length of the dam: on the contraiy, 
it will be necessary at certain.points to lower the gates to the bottom, 
with their tops below the water level so that the water may return to 
the river, but not along the bottom where it might carry sand. 

The arrangement of the dam will, therefore, be made along the 
following lines: starting at the upstream end, a section of fifty meters, 
in which the gates touch the bottom while their tops are above the 
level of the water: then fifty meters, in which the gates are raised 
above the bottom while their top is above the level of the water; 
finally, one hundred meters, in which the gates touch the bottom 
while their top is lowered below the water level. And so on in suc¬ 
cession in sections of two hundred meters. 

As the gates may be regulated, these arrangements might be alter¬ 
nated, and after a certain time the one hundred final meters of length 
might have the same arrangement as had formerly the first one hun¬ 
dred meters of length and reciprocately. In this manner scour would 
take place in front of the entire length of the dam, and the filling 
behind it would be effected readily. 

With what speed will this filling take place? It is difficult to 
prognosticate in advance. The reclamations effected hitherto on 
other rivers like the Rhine can give only insufficient data, for in all 


20 


these works open sluices for the passage of the sand are required to be 
left in the artificial banks. The presence of the dam with suspended 
gates will perceptibly accelerate the movement of the sand. It might 
be foreseen that after one whole year or perhaps two years, the filling 
will have attained such a height that the dam may be replaced by 
riprap. It will then be necessary to employ the usual methods to con¬ 
tinue progressively the filling at this portion of the bank until it 
reaches the height of the neighboring natural banks. 

Where the concavity of the bank is insufficient, the regulation is 
completed by the use of spurs attached to the convex bank. 

This first work will be completed as soon as possible, before the 
great floods of winter, because it is during the flood season that the 
filling may be pushed to the greatest height possible. 

This first regulation will result in a localization, more or less regu¬ 
lar, of the sand banks in the neighborhood of the convex portion of 
the banks; it will cause the existence of deep and regular pools along 
the concave banks where the current will be concentrated, but it will 
allow the sand bars at the points of inflection to continue. 

Dams with suspended gates will then be put in place to force the 
channel to pass from one bank to the other through a narrow channel 
hollowed out through the sand bars, or to effect the separation of the 
current at the entrance of the false channels. Traced so as to con¬ 
tinue the concave curve where the bank ceases to have sufficient curv¬ 
ature they will result in conducting through the sand bar a deep and 
narrow channel, and at the same time they will cause the elevation 
and regulation of the sand bars situated in front of the convexities. 
The gates will be put in place at the end ot the high winter floods, 
at the beginning of the mean water stages, which M. Jacquet has called 
the regulating stage, and they will remain in place during the period 
of low water. They will be taken away in the autumn before high 
water in order to leave during the winter a free flow for the flood 
waters. 

In order to leave a free passage both for ice cakes and other floating 
bodies, the pile dams may be arranged with as large openings as will 
seem necessary. The floating supports may be taken away when de¬ 
sired. 

After taking away the gates the current will preserve sensibly the 
same direction, since we have taken care to make the thalweg during 
low water coincide with mean direction of the current during high 
water. Therefore no sensible change in the position of the sand bars 
will take place; there can be only a slight displacement of sand, since 


21 


the principal force of scour will make itself felt in the concave curves 
which will have been entirely protected against erosion. The sand 
bars will receive a part of the sand in suspension and a partial filling 
in the channel will be produced, which will be easy to get rid of the 
following spring by replacing the gates in front of the piles which 
have been left in place, or in front of the floating supports which 
have been put back in their place. 

Thus even during high water the thalweg will preserve a direction 
which is almost permanent, and navigation may be continued without 
change. But this permanence in the direction of the thalweg must 
have on the longitudinal profile an influence which must be examined. 
In the portions where the river has only one branch, the sand bars are 
scoured away only on account of the presence of the dam; it will, 
therefore, be possible to arrest the scour at these points within any 
desired limit. Moreover, after the removal of the gates a partial filling 
will always take place each winter. Consequently there is no fear 
that the bed will be eroded in a progressive manner to an exaggerated 
depth. 

In localities where the river has several arms, and where the current 
is concentrated in a single narrow arm, the bottom will be constantly 
subjected to the scouring force of the current, and the bed will be 
hollowed out in a progressive manner. It will probably become neces¬ 
sary to limit this deepening by the use of ground sills. But as these 
little arms will have a breadth of only one hundred to one hundred 
fifty meters the establishment of the ground sill will be less costly 
than would be the case if it were necessary to use it across the entire 
river over a breadth of four hundred to five hundred meters. It will 
even be advantageous to transform these ground sills into reservoir 
dams which will permit the level of the low-water plane to be raised, 
and will furnish a motive power more or less considerable, depending 
on the discharge and the fall. But this latter is a complement which 
is not necessary, at least in the beginning, for the establishment of 
the navigable channel. 

At the entrance to the small arms the submerged separation dams 
will be established only after the deepening in front of the entrance 
has been effected. We should content ourselves at first with the con¬ 
struction of a dam with suspended gates at the head of the island. If 
sand enters the small arms it will not be deposited if their width and 
curves are regulated. 

It is important to know that our bank regulation works apply to 
natural banks, and when we substitute for the natural bank an arti- 


22 


ficial work, this work is always raised to the mean level of the banks; 
that is to say, from three meters to three and a half meters above the 
low-water stage. This is an essential condition in order to obtain 
the anticipated scouring effects; for it is only at the time of mean 
discharges that the channel can be regulated in a durable manner, as 
has said M. Jacquet in the report already quoted. 

On the Loire when the water falls to one meter and a half above 
the low-water stage the velocity of the current is scarcely one meter, 
and the scouring force is no longer very considerable. 

It is during the two to three meter stages of water, of which the 
duration is quite a long time, that the scouring effects intended to 
direct the channel must be produced. 

Other reasons, moreover, lead us to condemn low works which will 
be submerged for too long a time. As soon as an oblique wall struck 
by a current is submerged it necessarily ceases to be the cause of a scour. 

This fact was observed very clearly on the Loire in 1897. Above 
Ingrandes the right bank between stations 87 and 88 is markedly con¬ 
cave, and there is ordinarily at this point a deep pool. Now, during 
the summer of 1897 there existed there a very high, firm sand bar. 
What could be the cause of this anomaly? Its causes were as follows: 
during the winter of 1896 and 1897 the Loire had several very great 
floods (the levee on the left bank between Montjean and Saint Florent 
was carried away). During these successive floods the water rose 
more than four meters and the water, passing above the natural bank, 
was thrown directly against the railway bank at Orleans. There was 
thus produced a concentration of motion and scour; so the little ditch 
which follows the railroad was deeply hollowed out, while the natural 
bank between stations 87 and 88 formed only an obstacle to the flow 
of the bottom waters and consequently caused a deposit of sand. 

The theoretical explanation of this phenomenon is apparently quite 
simple. A wall struck obliquely by a current becomes a center of 
scour only on the condition that this current is intercepted in its 
upper layers and up to the surface. In this case, in fact, the exposed 
wall receives on its upstream face a pressure exceeding the hydrostatic 

V 2 

pressure by the quantity There results from this a difference of 

level: the surface of the water is lifted along the wall by a height 

V 2 

corresponding to This difference of level produces an increase 

of slope between the points struck and the neighboring points, and 
hence an increase in velocity. It is this increase in velocity added 




23 


to the increase of pressure which produces the scour on the bottom. 
But if the wall exposed to pressure does not extend to the surface the 
difference of level is much less, because the surface layers spread out 
immediately in the neighborhood; therefore the increase in velocity 
is produced specially at the surface, and is almost insensible in the 
lower layers. If, moreover, the exposed wall reaching to the bottom 
of the bed has a direction such that the liquid filaments at the bottom 
have their flow opposed and their velocity diminished, there will be 
a tendency to deposit. 

Whence we must conclude that the banks which we wish to make 
centers of scour must be rarely submerged, and especially must we 
avoid placing on the bottom of the bed submersible works whose 
direction is almost perpendicular to the direction of the current in 
high water because these submersible works will become, in winter, 
centers of deposits which the summer waters will not succeed in re¬ 
moving. 

There is still another advantage in regulating natural banks in that 
they are thus protected against the destructive effects of the current; 
consequently the transportation of sand in the river will be diminished, 
and the property of riparian owners will cease to be exposed to the 
encroachments of the river. 

To sum up, the method proposed for the improvement of rivers 
with erodible bottoms consists essentially in requiring regulation of 
the banks of the mean bed, completed by the construction of spurs 
attached to the convex bank. This improvement, though permanent, 
is only partial and would be insufficient in low water. The improve¬ 
ment is then completed by the construction of an artificial channel 
through the bars, but only during low water. 

The means proposed to create this channel consist of a particular 
arrangement of gates on a plan oblique to the current. The oblique 
dam with suspended gates constructed on this association of ideas 
produces a double effect: on the one hand,it causes a rapid scour fol¬ 
lowing a definite direction; on the other hand, it produces the sepa¬ 
ration of the current of the lower layers containing almost all the 
sand from the upper layers of the current which carry no sand. These 
two portions of the current take different directions, the lower layers 
carry the sand outside of the channel, while the upper layers, in more 
considerable volume, are concentrated in the direction of the channel 
which we wish to reserve for navigation. 

This phenomenon may be utilized for the improvement of rivers in 


24 


several very different ways, and its application is not limited only to 
the improvement of the Loire. 

The advantages which an application of this system would present 
in the case of the Loire appear to be as follows: 

This system, as we have just seen, protects the banks against 
erosion by the current, since all the concave banks are covered with 
riprap. As a result, riparian properties will no longer be in danger 
of being carried away by the river, and in this there is a considerable 
gain to the public, since M. Krantz has been able to calculate that 
the volume of earth carried away each year by the Loire lies between 
500,000 and 1,500,000 cubic meters. 

In the second place, this system is adapted to obtaining great drafts 
of water. It is an assured fact that the greater the curvature of a 
concave bank the deeper will be the pool which lies along it. We 
may, therefore, choose the curvature which is adapted to the depth 
we wish to obtain, and may make the effort to give to the banks a 
trace or plan which resembles this curvature. 

Moreover, for the passage of the current from one bank to the other 
it is possible, by varying the trace and length of the dam with sus¬ 
pended gates, to increase the depth of the channel. 

It must, moreover, be noticed that great depths of water may be 
obtained without modifying the elevations of the general water levels 
of the river: for they are produced by accentuating the dissymmetry 
of longitudinal profile without sensible modification of the wetted 
perimeter, nor of the mean level of the bottom. 

On the other hand, there is one condition which every system of 
river improvement must fulfil: it must be adapted to changes which 
experience may suggest, and must not create any irremediable con¬ 
ditions. 

The new system which we propose appears to lend itself better than 
any other to successive corrections, since to remove a dam with sus¬ 
pended gates it is only necessary to pull up the piles and place them 
in another position. 

As to the net cost, it appears it ought to be lower than methods 
hitherto employed, for, according to an approximate calculation, the 
total expense of the construction of a navigable channel between 
Angers and Nantes would amount, under this system, to only ten 
million francs. 

Finally, this system permits the ultimate establishment of transverse 
dams with heads of water, and, consequently, the creation of motive 
power. 


25 


We have seen above that permanent passage of the current in cer¬ 
tain narrow arms might lead to a progressive deepening of these arms. 
It might become necessary to limit this deepening by a bottom sill 
placed at the downstream end of the arm. It will, also, be advanta¬ 
geous to utilize the bottom sills as reservoir dams. This is an advan¬ 
tage upon which it is important to insist. 

As we have already shown, these bottom sills will be necessary at 
first only in narrow arms the width of which will be only one hundred to 
one hundred fifty meters: consequently, the establishment of these 
bottom sills with reservoir dams will be easy, without requiring an ex¬ 
travagant expenditure. If, in fact, conditions of construction similar to 
those existing at the Poses Dam are admitted, that is to say,a net price of 
twelve thousand five hundred francs approximately to the meter, a dam 
one hundred to one hundred fifty meters long will cost from one mil¬ 
lion two hundred fifty thousand to two million francs. Now, at 
Poses the reservoir dam head is four meters. Each one of the trans¬ 
verse dams to be built on the Loire would have a head of only one to 
one and a half meters, and might consequently be constructed under 
more economical conditions. The minimum motive force to be an¬ 
ticipated at each dam is that which corresponds to the minimum dis¬ 
charge of the river at low-water stage, namely, one hundred twenty- 
five cubic meters per second. For a fall of one meter the corresponding 
theoretical force would be 1600 H. P. The value of the utilization 
of this force is sufficient to fully pay the interest on a capital of two 
million francs. The navigable channel would, therefore, result from 
this work without cost to the state. We have already noticed the 
advantage which the navigable channel must secure from the estab¬ 
lishment of ground sills, viz: the stability of the longitudinal profile. 
The addition of reservoir dams will bring other advantages for navi¬ 
gation, by diminishing the current, and by permitting the plane of low 
water to be raised so that without having need to seek exaggerated 
deepening in the channel, we may have always a sufficient depth of 
water for navigation. 

Finally, the creation of motive forces along the navigable channel 
communicating with the sea will cause in the valley of the Loire an 
industrial activity and marvelous prosperity. 















■ 







The Control of Rivers With 
Unstable Beds, With Special 
Reference to the Loire 

by 

M. E. AUDOUIN 

Professor in the University of Poitiers 














The Control of Rivers With Unstable Beds, 
With Special Reference to the Loire. 


The improvement and development of internal navigation, which 
is the indispensable auxiliary of railroads and maritime navigation, is 
at present engaging public attention both in France and abroad more 
than ever before. 

But if an agreement has been reached as to the utility of navigable 
waterways, opinions differ as to the means for securing them. 

It is often a question of deciding which of three methods to take: 

1. Improvement by Regularization; 

2. Improvement by Canalization; 

3. Construction of a Lateral Canal. 

LATERAL CANAL OR RIVER IMPROVEMENT. 

In a canal, the depth is fixed and traction is as easy in one direction 
as in the other. But, on the other hand, the cost of constructing a 
canal is generally very great, particularly if it is desired that the canal 
be capable of handling a large traffic, which necessitates locks of 
great size. Moreover, a lateral canal serves only one side of a stream. 
Hence, for economical reasons and in order to respect, as much as 
possible, the rights of people living along a river, a lateral canal 
should not be resorted to, except in a case where the regimen of the 
river, its condition of slope and flow in a given part of its course, 
such as the upper part of the valley, do not lend themselves to any 
other method of securing a navigable waterway. 

M. Barlatier de Mas, Inspector-General, in his report to the First 
National Congress of Internal Navigation at Bordeaux, states as fol¬ 
lows: ''Logically, river improvement should precede the construction 
of canals, the latter having as their object the prolongation of the 
navigable waterway, in the higher part of the valleys, where the natural 
waterways can not be used (lateral canals), or to connect different 
river basins (summit-level canals).” 

The first method of river improvement to consider is regularization. 
At the International Congress on Stream Utilization, in Paris in 1889, 



30 


✓ 


M. Boule, Chief* Engineer, very properly stated: ”In most cases, 
works of regularization are indispensable, for, before a river can be 
canalized, the course and banks must first be fixed.” 

PROJECTS FOR THE IMPROVEMENT OF THE LOIRE. 

In view of the above, Parliament acted wisely in reference to the 
Loire improvements, when, in passing the National Utility Act of 
December, 1903, it ordered the immediate execution of certain pro¬ 
jects for the improvement of navigation in the vicinity of Angers. 

These works have just been completed and their effect is already 
quite noticeable. Some of these works consist of epis* similar to those 
which have been constructed on the Rhone, the others consist of 
oblique dams with sluice-gates. 

The first system is well known, while the second, on the contrary, 
is quite new and, as it has given better results, a description of it 
would seem to be more proper. 

OBLIQUE DAMS WITH SLUICE-GATES. 

The system of oblique dams with sluice-gates depends on the fact, 
that in all streams with unstable beds, the movement of sand and 
gravel by the current is principally on or near the bottom. The ob¬ 
ject of the system is the regularization of the stream by an automatic 
decantation, in which the upper layers of the current are concentrated 
in the channel so as to deepen it by scour, while the lower layer, 
passing under the gates of the dam, carries the sediment beyond the 
channel. 

It is well known that, according to the natural laws of the regimen 
of rivers with unstable beds, as established by M. Fargue, Inspector- 
General of Ponts et Chaussees, the channel always forms along the 
concave bank when this bank is regular and without sudden change 
of curvature and the succession of curves is properly suited to the size 
of the bed. In order to assure the constant formation of pools, the 
banks, and particularly the concave banks, must first be prepared by 
replacing the irregular natural banks with artificial banks of gradual 
curvature. When the position of the pools has once been established, 
it remains to deepen the channel on the bars that form naturally at 
points where the current changes direction, i. e., where concavity 
changes to convexity. 

This double result can be secured rapidly and economically by 


*Low spur dikes. 



LS ¥ 


PI. I. 


Plan de sondage leva avant implantation de l’ouvrage 

(Sondage execute les 19, 20, 21, 22 et 23 septembre 1903) 



jB<3Tjya<^e sSjOsLeme /Iuc/oui.n 


( .EcAeJSe c7e 1 ) 

1 6ooo*) 


Plan dressd par i’Adininistration des Ponts et Chaussdes (Service special de la Loire, 4* section'; 

M. Cuknot, ing^nieur'en chef; M. Philippe, ing^nieur ordinaire). 

Nola. — Les chldrcs placds a cdld des courbes de niveau indiquent les profondeurs au dessous de I’dtiage. 
Les grftves s’dlevant au-dessus de I’etiage sont indiqu^es par un sablA 














































■ 

?■ 













31 


means of oblique dams with sluice-gates, as has just been demon¬ 
strated by the new works on the Loire about three kilometers above 
the junction of the Maine and near Port Thibaut. The project for 
these works was prepared by M. Chemin, Chief Engineer, and was 
approved by the Minister on the 26th of August, 1905. 

CONDITIONS PREVIOUS TO COMPLETION OF IMPROVEMENT 

(PLATE I). 

The dam is located on the left bank of the Loire, at the outlet of 
that arm of the river which is south of the lie aux Dames. This 
arm, which begins at Ponts de Ce, has a mean width of 150 meters 
and receives the larger part of the low-water flow of the Loire; con¬ 
sequently it has, as a general rule, a fair channel. But, near the 
outlet, its width becomes excessive, and the left bank, which to within 
200 meters of K. 558 is sufficiently concave to maintain a good 
channel, changes sharply at this point to convexity. For this reason, 
a bar formed in the middle of the bed, and, as can be seen by the 
chart of soundings taken on September 19 to 23, 1905 (Plate I), the 
crossing was very bad before the works of improvement were com¬ 
menced. A pool extended along the left side of the stream near the 
bank, and on the right side, a pool existed along the lower part of the 
lie aux Dames, the crossing from one to the other being made across 
stream and in depths that did not exceed one-half meter during the 
low-water stage. 


DESCRIPTION OF THE WORKS. 

The dam starts at the point H (Plate I) where the change of curv¬ 
ature of the left bank takes place. Its trace is a curve, which is a 
continuation of the concavity of the left bank, and it is directed 
towards the pool which is near the right bank of the river between 
the points K’s 59 and 60. The annexed photograph (Fig. 1), taken 
from near the upstream end, shows the curved form of the dam. It 
has a total length of 450 meters and consists of two parts, as set forth 
by M. Chemin in his project: 

"The first part, 250 meters in length, is a rectification of the left 
bank of the stream. It is tangent to the bank at the point where it 
joins it, and prolongs the bank into the widened stream bed.” 

The second part of the dam is 200 meters in length and is in¬ 
tended to cause the formation of a channel of sufficient depth across 
the bar at the point where the current changes direction. 

Both parts of the dam are of similar construction. They consist of 


32 


a single row of piles of 8 meters average length and 30 centimeters 
average diameter, spaced 2 meters and so driven that their tops are 2.25 
meters above low-water mark. The tops of the piles are joined by 
two courses of waling pieces, 20 by 10 centimeters, bolted to them. 
A third row of waling pieces, which serves to hold the lower ends of 
the upright guides, consists of pieces 4.50 meters in. length, each of 
which is fastened to three piles. As this third course was put in 
place when the stage of the water was 2 meters above the low-water 
stage, they could not be bolted to the piles as were the upper courses 
and were hence fastened to the piles by means of metal collars. This 



Fig. 1. View of Entire Dam. 

latter course was placed 2 meters below the upper courses, and before 
each piece was put in place there were fastened to it the four wooden 
guides which it was designed to hold. These guides, which are 
spaced 1 meter apart, were then fastened to the upper waling courses. 
They are for the purpose of guiding the wooden gates, which are 4 
meters high, .90 m. wide and .02 m. thick, and provided with 
uprights, so that they may be raised or lowered easily by hand. Fig. 2 
shows a man in the act of raising a gate by hand. The gates are held 
at the desired height above the bottom by means of loose bolts which 
are inserted in holes in the uprights and rest on the upper waling 
courses. 

The gates of the first part of the dam were put in place on March 











Plan de sondage leve trois mois apres Pachevement de l’ouvrage 

(l)u 14 au 16 aodt 1906) 


PI. II. 



Nota. - Las chi Area placds a c6te des courbes de niveau indiquent lea profondeun aiwlj*‘Ifent “."KnUaur 
exceptd ceux qtri se trouvent dans les parties pointings reprdsentant les graves , ces dern q 

au-dessus de l’^tiage. 





























































1 



' 





■ 


















33 


29, 1906, when the stage was 2.25 meters above low water, while 
those of the second part were put in place on May 16, 1906, the stage 
at that time being only 1 meter above low water. 

RESULTS OF THE EXPERIMENT WITH OBLIQUE DAMS WITH 

SLUICE-GATES. 

The results, as shown by the chart of soundings taken on August 14 



Fig. 2. Maneuvering the Gates. 

to 16, 1906, three months after the completion of the work, are as 
follows (Plate II and Fig. 3): A channel, at least 60 meters wide, 
had already been scoured out along the first part of the dam, with 
the following depths at low water: 

Cross-section 557 H (left bank), 2.20 meters. 

Cross-section 557 I, 2.10 meters. 

Cross-section 558, 1.60 meters. 

Cross-section 558 (left bank); 59 (right bank), 1.40 meters. 





















34 


A very large amount of sand had passed under the gates and had 
been deposited between the dam and the natural bank in a height of 
more than 1 meter. 

Along the second part, which had been put in place two months 
after the first and in quite low water, the scour was not so great; 
nevertheless, a depth of 0.70 m. had already been attained in August, 
1906. The channel was deviating from the dam in such a direction 
as to join the pool which existed along the lie aux Dames and had a 
low-water depth of more than 1 meter. 

During the winter of 1906-07, the gates were left in place, but 
raised as high as possible, and the dam sustained the test of high 
water and ice without damage. 

The chart of soundings taken on June 19 and 20, 1907, shows that 
the channel follows the dam throughout its entire length, with a 
low-water depth of not less than 1.50 meters and that it is being ex¬ 
tended downstream toward the pool on the right bank of the river 
between the points K. 59 and K. 60 (Plate III). The pool which 
formerly existed along the lower part of the lie aux Dames has dis¬ 
appeared, as had been foreseen; since the dam prolonging the concave 
curvature of the left bank would necessarily retard the change of 
direction of the current. This was a secondary pool which it was 
advisable to dispose of. The flow is now concentrated in the middle 
of the bed, across the bar which formerly obstructed it and which in 
1905 extended above low-water mark. The sand has been diverted 
to one side. The sand bank, which formed behind the dam in 1906, 
is extended downstream along the left bank, where it aids in limiting 
the channel below the dam and giving it a good direction. 

It will be noticed that the effect of this dam is felt for 200 meters 
below its downstream end. For 150 meters the channel has a low- 
water depth of 1.50 meters and holds the direction which the dam 
gives to it, while for the remaining 50 meters, or the distance to the 
pool along the right bank, the depth as yet is only 1.30 meters below 
low-water mark. 

RESULTS OF IMPROVEMENT WITH EPIS. 

It is interesting to compare the results of this work with those of 
works of improvement by means of epis, which were constructed on 
the Loire below the confluence of the Maine. An examination of 
the official charts of soundings, on exhibition at the Exposition of 
Bordeaux, and which show conditions both before and after the con¬ 
struction of the works of improvement, would make evident that epis 


Profils dresses par TAdministration des Ponts et Chauss6es 


Service special de la Loire 
4* section 


Tlche/]eJ 

JjO/igiicu/'-f O ”002 pour /O ine/j c-/ 
f/ctuleurJ O'" O pour J rneire 


LEGBNDE 

—-Fond du ileuve avant implan¬ 

tation de i’ouvrage. 


- Fond du Ileuve en aoul 

Irois mois apres Fachevement de l’ou- 
vrage. 


Nota. — Les cotes indiquent la pro- 
fondeur au-dessous de 1’etiage. excepl6 
relies qui sont prec^dees du sigoe } 
(derri6i»e le barrage) ; ces derni6res 
indiquent Ja hauteur au-dessus de 1*6- 
tiage. 





Fig. 3. — Prolils en travers de 1’ouvrage. 
























































































































. 

































































are not without value in certain ways and that a channel is being 
scoured out in places. But what is most important and must be most 
considered is the existence o: a continuous channel and the minimum 
depth of water over all of the improved section of the river. New. 
in the branch of Chalonnes. as shown by the charts of soundings, a 
number of crossings exist when the channel has a low-water depth of 
less than O.SJ m. Likewise, in the part between the lie de Chalonnes 
and the He de Behuard. there are several bars on which the low-water 
depth does not equal 1 meter, though the works have been completed 
for three years and have had time to produce their etiect. At least 
such was the case last April which was the time when the data for 
the latest chart of soundings was secured. It is possible that this 
condition of affairs may have been improved upon since that time, 
due to the extensive dredging that it has been necessary to resort to. 
Dredging, however, will not secure a permanent channel and. as it 
must be often repeated, the cost of a channel by such means is great. 

ADVANTAGE OF OBLIQUE DAMS WITH SLUICE-GATES. 

One of the advantages of oblique dams with sluice-gates is that 
dredging is not required, since the effect of the dam itself is a sort c: 
automatic dredging. 

Moreover, the results, so far as the regularization of the banks is 
concerned, are more rapid than with any other method, and this is c: 
the highest importance, no: only for navigation, but also for agriculture. 

"The t.ring %f the batiks, states M. ce Mas in the report a.ready 
quoted, "which is an essential part of works or regularization anc 
which with us in this later system has generally preceded canalization. 
assumes the anseruatun cia ever, the increase »/ 'latiwuai resources; con¬ 
servation in putting an end to the erosions which destroy valuable 
lands, and increase, in permitting the utilization or .arts the cut - 
vation of which had previously been impossible. 

The oblique dam with sluice-gates not only regularizes and protects 
the banks, by substituting for the irregular natural bank one that s 
artificial and "egular and less liable to eros:on. rut a.so causes the 
rapid rilling up of the interval between the old bank and the new. 
Hence, in addition to protecting the property c: riparian owners 
against the action of the current, it causes the stream to reform n a 
short time important areas which it had previously destroyed. There 
is. therefore, to use the expression of M. de Mas. an increase of 
national resources, sufficient to justify worcs of this character even 
when the question of improving navigation is not considered. 


36 


As is well known, most of the works of regularization of rivers in 
Germany, as well as those on the Garonne, have been executed in the 
interest of agriculture as well as navigation. M. Robert, Chief 
Engineer, says on this subject in a report to the Eighth International 
Congress of Navigation in Paris in 1900: 

"The works of regularization on the Garonne, undertaken in the 
interests of both agriculture and navigation, are for the purpose of con¬ 
centrating all the flow, except at a time of inundation, in a single 
bed with fixed and regularly traced banks.” 

METHOD OF REGULARIZATION.' 

In order that works of regularization may be beneficial to agri¬ 
culture they must be applied to the mean bed, that is, the area covered 
by water when the stream flows full to the banks, and not to the 
minor bed, which is the area occupied by the stream at the low-water 
stage. The low ground between the banks of the mean bed is 
covered by water the larger part of the year and only in rare instances 
is suitable for cultivation. Hence, the banks of the mean bed are 
those most suited for regularization and protection, in order to pre¬ 
vent erosion and to recover land. The banks of the mean.bed of the 
Garonne are those to which the remarkable works of regularization 
of M. Fargue, Inspector-General, were applied. On the Rhone also, 
the work of regularization consisted first in the rectification and 
establishment of the mean bed bv the use of longitudinal dikes. 
M. Jacquet, Inspector-General, who had charge of the works of im¬ 
provement on the Rhone before M. Girardon, Chief Engineer, says 
on this subject in a report to the International Congress on Stream 
Utilization at Paris in 1889: 

"Dikes should concentrate the water of stages that are sufficiently 
high to have an efficient action. The height adopted for the crown 
of the dikes on the Rhone was 2 meters above the low-water stage 
between Lyons and Isere, 2.50 meters between Isere and Ardeche and 
3 meters between Ardeche and tidewater.” 

M. Jacquet has remarked that the action of the current upon the bed 
of a stream depends on its volume, and with a given volume, upon its 
duration, and further, that the state of the bed that is most permanent is 
due to mean volumes, sufficiently strong to act sensibly on the bottom and 
yet durable enough for this action to be efficient. 

“ Neither floods nor low-water flow,” he states, “give to the bars and to 
the bed of the stream the forms and the relief that are noticed at the low- 


Plan de sondage lev6 les 19 et 20 juin 1907 


PI. III. 





































































































U ’ 






















37 


water stage. The effect produced by floods is modified by the lower 
stages which follow, and which determines the definite form that remains 
after each flood. These intermediate stages are the real regulators of the 
low-water bed.” 

Since the mean volumes of flow determine the shape of the minor bed, 
it is to the banks of the mean bed that works of regularization should be 
applied. 

The works of M. Girardon on the Rhone are applied to the minor bed 
and the same is true of the epis which have just been placed on the Loire, 
between the confluence of the Maine and Chalonnes. The two types of 
works which have just been completed on the Loire differ, then, not only 
in construction, but also in the method of regularization; the works at 
Chalonnes being an example of one method, that of M. Girardon, while 
the recent works at Port Thibaut exemplify the method of M. Fargue. 
In the first, it is desired to fix a channel in the minor bed by means of low 
works, without regularization of the banks themselves, the channel cross¬ 
ing from side to side with many curves. In the second method, a con¬ 
cave or directing bank of the mean bed is regularized by means of a high 
work, and at the change of direction of the current a channel is formed, 
following the direction the current of the mean stage 'would take when 
directed by the regularized concave bank. 

Experience seems to have demonstrated that the second method is pref¬ 
erable to the first both for navigation and for agriculture. It is confirmed 
by the opinion of such eminent engineers as *M. de Timonoff and the 
members of the Mississippi River Commission, according to whom “no 
improvement of the minor bed should be attempted so long as the direction 
of the high-water flow is not established.”' 1 ' 

COST. 

The method of improvement, tried near Port Thibaut, is not only better 
for navigation and agriculture, but is also more economical. The section 
of the river which has been improved by means of these works and on 
which a good channel has been secured has in all a length of more than 
2 kilometers. This result has been secured with an expenditure of about 
30,000 francs. If, in order to determine the cost of this system of im¬ 
provement, only the distance between the centers of the pools joined is 
taken into consideration, the cost would be 30,000 francs per kilometer. 
According to the project for the regularization of the Loire by means of 
epis the cost would be more than 100,000 francs per kilometer. Hence, 

^Report to the Eighth International Congress of Navigation in Paris in 1900, 
page 180. 



38 


with the system of dams with sluice-gates, the cost would be less than 
one-third as great, and the Loire could be made navigable from Nantes to 
the confluence of the Maine for less than 3,000,000 francs. It should 
not be difficult to secure such a relatively small sum for so important a 
work. Parliament, after having appropriated 1,600,000 francs for works 
of improvement over a length of 12 kilometers, should not hesitate to ap¬ 
propriate a sum twice as large to make the Loire navigable over a length 
of 84 kilometers, from Angers to Nantes. The easiest way to secure this 
amount would seem to be that as the works are authorized the depart¬ 
ments interested be requested to furnish one-half of the amount necessary, 
while the government furnishes the other half. 

When the Loire has once been made navigable from Nantes to Angers, 
which is the most urgent, work of improvement could then be carried on 
upstream as far as Vienne and finally to Tours and Orleans. 

STORAGE RESERVOIRS. 

According to a report of M. Schwob to the National Congress of Navi¬ 
gation, the Loire could be made navigable as far up as Briare by means of 
storage reservoirs. 

“In order to control and utilize the Loire as far as Briare,” says M. 
Schwob, “ there must be an increase of the low-water flow of 40 cubic 
meters per second. After the great floods of 1857, M. Comoy, Inspector- 
General, made a rapid study of storage reservoirs for storing a part of the 
water during floods. Sites for these reservoirs were selected and the cost 
of construction calculated. The cost of works storing six hundred million 
cubic meters of water was found to be sixty-five millions of francs. M. 
Comoy estimated that this cost would be largely compensated for by the 
security against floods that would thereafter be furnished to the inhabitants 
and property owners along the river. Nevertheless, the Imperial Govern¬ 
ment could not provide the requisite funds. At the present time condi¬ 
tions are much better. Due to the rapidity of telegraphic communication, 
the open storage reservoirs of M. Comoy could now be changed to closed 
reservoirs, which could be drawn on at the least change of stage. 

'Therefore,” continues M. Schwob, '‘they could be used: 

“L As sources of supply during low water, increasing the amount of 
flow from 70 to 80 cubic meters per second during the low-water period, 
which is double the amount necessary for purposes of navigation. 

“2. As a source of motive power, permitting the establishment of in¬ 
dustrial plants in the country surrounding Saint Etienne, Montlugon, Cler¬ 
mont, and as far as Bourges. 

‘'3. Finally, since the amount of water stored would be much greater 


39 


than that required for purposes of navigation, part could be devoted to 
agricultural purposes after it had passed the turbines.”* 

Storage reservoirs alone are not sufficient to make the Loire navigable, 
but by acting as regulators in diminishing the ratio of the volumes of 
flow at high and low water stages they would improve the regimen of the 
stream and would permit works of regularization to have their best effects 
both in the lower part of the Loire and as far up as Orleans.f 

COMPARATIVE COSTS OF REGULARIZATION AND A 

LATERAL CANAL. 

Omitting the cost of storage reservoirs, which would be largely com¬ 
pensated for by the profit accruing to manufacturing establishments and 
agriculture, an expenditure of twelve millions of francs would make the 
Loire navigable from Nantes to Orleans. 

According to an estimate made recently by the administration of Ponts 
et Chaussees, a lateral canal from Nantes to Orleans would cost about 
one hundred and ninety millions of francs. Such a sum, in the opinion of 
the Paris Chamber of Commerce, seems out of proportion to our resources. 

Works of regularization will cost little and offer, as has been shown, the 
advantage of serving both navigation and agriculture. 

ADVANTAGES OF CANALIZATION FOLLOWING 

REGULARIZATION. 

The very important results which are procured at first by works of reg¬ 
ularization may be completed later by canalization. An eminent engineer, 
who has a good knowledge of conditions on the Loire, M. Lechalas, In¬ 
spector-General of Ponts et Chaussees, has always maintained that the 

*The Eighth International Congress of Navigation, which met in Paris in 1900, 
reached the following conclusion on the advantages of storage reservoirs: 

“ One of the means that are available for the improvement of the regimen and the 
navigability of rivers is the use of storage reservoirs for increase of the flow at low- 
water stages and also in certain cases to reduce the height of floods. Storage reser¬ 
voirs have been used with success, and others are being planned, and this means, 
which has the additional advantage of being a source of power that can be made 
available through the achievements of science, is commended to the attention of en¬ 
gineers and governments.” 

f I had the honor of calling the attention of the General Council of Maine-et- 
Loire, during the session of August, 1906, to this important subject of storage reser¬ 
voirs, and at the same time to the necessity of taking immediate measures to prevent 
sediment from the basin of the Allier being carried into that of the Loire. This was 
in accordance with a resolution adopted by the Twelfth Congress for the improve¬ 
ment of navigation on the Loire, and the General Council of Maine-et-Loire adopted 
a resolution on the subject. 



40 


Loire can be made a good navigable waterway by regularizing it at first 
and finally canalizing it by means of movable dams. 

No better navigable waterway exists than a river canalized like the Seine, 
since its capacity for traffic is much greater than that of an ordinary canal. 
Another advantage of a canalized river is that motive power may be gen¬ 
erated at the dams, which is available for towing boats and for industrial 
establishments along the river. The benefits arising from these hydro¬ 
electric installations, which are now being introduced everywhere, would 
tend to compensate for the cost of construction of the navigable waterway 
and would aid its operation. 

I would add that, for the Rhone, which has been regularized, the best 
thing to do now is to canalize it, instead of constructing a lateral canal, 
which is under discussion. 

Future development will be in the direction of the canalization of rivers, 
large locks being provided, which will permit the movement of fleets of 
boats, and boats of large tonnage, the power necessary for traction being 
generated at the dams. 

But canalization supposes a previous regularization. It is eminently 
proper, then, to commence the improvement of the Loire by regularizing 
it and making it navigable at a small cost. Then, when once the boat 
traffic of the Loire has been restored, the depth can be gradually increased 
and the expenditures regulated as the growing needs of traffic demand. 

E. Audouin. 


























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